I know what you did last Halloween
by lotrharrypotter3
Summary: <html><head></head>Kind of AU in which TMI characters decide to celebrate Halloween in the Institute while Lightwood are away (Only the adults and Max of course, Izzy and Alec are here. xD It would be boring otherwise.). A lot of unexpexted situations happen to them in this ought-to-be-a-fun-night. Enjoy (First chapter is only intro, the rest would be longer and filler.)</html>
1. Prologue

**A/N: So Halloween is coming soon and I had this idea where the TMI characters decide to celebrate Halloween in the Institute while the Lightwoods are away. I know the first chapter is really short but it's more like an intro than a real chapter. I'll post more very soon, I promise. Also it's kind of an AU. They are still Shadowhunters, Downlorlders, Demons and all but Simon is not a vampire yet, Clary and Jace aren;t told they're brother and sister and it's not during any of the books or events from them, nor between any of them but it's after City of Bones for sure! Everything else is the same. For now, enjoy :D**

**Disclaimer: I guess you won't believe me if I tell you I own anything except the plot so all I could do is admit that I don't :D **

"Are you sure about that?" asked Simon for about fifth time since his best friend told him about the evening. "You know that Jace-"

"But Isabelle is the one that invited us. And Jace doesn't have the word as she said." Interrupted him Clary, trying to assure him everything would be okay.

"That won't stop him from kicking me out, you know" muttered the boy quietly.

Clary only rolled her eyes at this.

"I promise you he won't do anything like this to you" she said.

"That's what you're saying" wouldn't give up Simon.

"Oh, come on, Simon! It's Halloween! It would be a great fun. Just relax"

"Why do I always hear that before something totally not-fun happens?" he asked sarcastic. "And stop rolling your eyes, you know I'm right." He added as Clary was about to do it again.

"Let's just go and see how it's going to be, okay?" she asked him "Besides, Luke promised he'll pick us up at anytime if we want to go home earlier."

"Okay, okay. But hey, don't forget I warned you about this."

"And don't forget, we'll be away for only three days so try not to set the Institute on fire, right?" said Maryse for a last time before she and her husband, Robert, leave with their smallest child, Max, for Idris.

"Yes mom, don't worry for us. We'll be just fine" said Isabelle as she smiled to her mother like an angel.

"Unless someone decides to make a Halloween party and invite half New York" added Jace, smiling mischievously, which brought him a death glare from his adopted sister.

For a moment it looked like Maryse was about to say something but Robert interrupted her.

"Oh, come on, Maryse. Leave the children have a little fun. I'm sure they won't do anything stupid or reckless." With those words, he dragged his wife along with him to the car, with the already waiting Max.

"Bye" said the others in unison, just waiting them to leave. As soon as this happened, Isabelle turned her head to Jace, her eyes burning holes at him.

"Are you nuts?! What are you thinking by telling them this?!" she asked with an angry voice.

"And what exactly are your thoughts, Isabelle?" asked her brother, Alec, glancing at her. "How do you think they will react if they find out you've decided to throw up a Halloween party while they're gone?"

"Oh, just calm down Alec, we're sure Izzy got the situation under control. Like every time when she has a crazy idea" said Jace with sarcasm.

"Of course everything would be under control." Told them Isabelle, calmly. "Besides, those are only a few people and we all know them already. Clary would be here, too." She said.

"Yeah, with her mundane friend." Said Jace, frowning.

However, Izzy just decided to ignore that.

"Magnus Bane would also probably come" she added, looking at her brother pointedly.

"What?" he asked maybe a little too quickly. "What is he going to be here for?"

Isabelle's smile just went wider as she once again said.

"It would be fun, I'm telling you"


	2. Welcome to the Institute

**A/N: Thank you so much for the follows/favourites and the review! Means so much!^^  
>Sorry this came later than I first thought and also, it's again, too short. Felt like it's a nice place to end However, hope you still enjoy it :D ^^<strong>

**Disclaimer: I do not own the characters, places, etc, but most of the plot in this piece of work is mine :D **

It was already dark when Clary stepped in front of the New York Institute door. Although she and Simon had spent the day together, shortly before they were about to go, he had told her he had a little work before he would come to the party. Clary was a little surprised as she thought they'll go together but agreed with him nonetheless. It was strange, she thought. Usually she spent Halloween only with Simon and her family. Her mother used to paint the pumpkins and they decorated the house, much to Madame Dorothea's dislike. Sometimes Luke came and they watched scary movies and Simon joined, too. Now it was so much different as Jocelyn was in coma and everything else. Clary's life was so different.

She slowly reached her hand and rang on the bell. As s Shadowhunter, she was able to get in without much effort but decided that being invited, it would be a little improper to break and enter just like that. It just didn't felt right.

After a few seconds, the door was opened by Isabelle. As always she was dressed perfectly, noted Clary. She was typical for the Shadowhunters black clothes, but there was also a pair of cat ears on her head. Her whip was around her arm, as always, but it was painted black, too. Costumes! Clary thought. She had totally forgotten about that.

"Hi, Clary. Ready for party?" said Izzy. She was cheerer than usual and too nice which made Clary wonder.

"Hey. I guess I am." Answered Clary a little constrainedly.

"Then let's go" said the other girl, moving from the door so Clary would enter.

Once in, she looked around. The Institute wasn't changed much but she noted that there were some pumpkin' lanterns and fake spider webs at the corners.

They entered not very big room. Not as like the rest of the Institute, this one was completely decorated. There were candles and lanterns, fake spiders in huge webs, bats and skeletons all around. The light was only from the candles and lanterns which made everything much more mystique and fitted perfectly to the decoration. In the middle of the room, there was a table and couches and armchairs. Alec was sitting on the edge of one of the couches, looking annoyed and Jace was sitting on a big armchair, smiling mysteriously at something which probably only he knew.

"Oh, hey, one more person came. This makes us four. Great party, right?" he said, his grin widening, as Clary and Isabelle came.

"Um.. hi" said Clary. She felt really nervous and regretted coming alone.

_I had to wait Simon do his 'work' so we would come together_ – she thought.

She and Jace weren't talking much lately. After all that happened, she couldn't say she didn't had any feelings for him but he was so impossible sometimes.

"Hi, Clary. How came that you're not with your mundane friend this time?" asked Jace mockingly.

"Jace!" Isabelle warned him. "Yes, where is Simon, I thought he'll also come." She turned to Clary.

"He said he had a little work but he'll come later." Answered Clary.

"Oh, great" said Jace with sarcasm and Izzy shot him a glare.

"And where is everyone else?" asked Clary. As Jace has said earlier, they were only four. Knowing Isablle, she expected at least twenty.

"There was no one else to invite. We can't invite mundanes and I don't know many downworlders that would come so…" trailed off Isabelle. "Thought if you ask Alec, this is big enough party." She looked at her brother, sighing.

"He's just grumpy because Magnus couldn't come." Added Jace.

"No, I'm not!" said Alec, who, by this moment, had remained quiet. "I just find the whole idea for party stupid. And also, Magnus said he probably wouldn't make it but he'll try to come."

"Yeah, right" said Jace but before Alec could argue again, Isabelle interrupted them.

"So, as we're the only ones here, maybe we should do something fun instead of just arguing with each other" she said, looking at Jace as a warning. "Watch a movie, maybe?" she added as nobody else seemed to have something in mind.

"Sounds fun" said Clary, who felt the need to join the talk more often.

"Why not." Added Jace.

Alec didn't spoke at all but nodded, which the others took as agreement.

"Great! So it would be a movie then." Said Izzy cheerfully. "Someone has any ideas?"

"Saw?" asked Clary.

"No way, it sucks. It's not even scary." Said Jace, which just made the girl roll her eyes. "So do you have any better idea?" she asked him to which he grinned mischievously.

"Evil dead!" he said.

"No! It's boring and we've already seen it too many times" said Alec suddenly.

"Oh, you're just still scared…" told him Jace as his grin grew even more.

"Hey, who hid behind me during the half of the movie?" asked Isabelle, protecting her brother.

Clary raised her eyebrows, not understanding what they were talking about.

"We watched it a few times but it always ended the same way." Explained Izzy as she saw her.

"I thought Shadowhunters weren't afraid from those kind of things" said Clary mockingly.

"When you're fighting demons, you think all about the fight, when you watch it on TV it's different" said Jace.

"Okay…" said Clary not very convinced.

"So… any other ideas?" asked Izzy.

"Insidious?" asked Alec.

"Whatever. I probably won't even watch it.. " answered Jace.

"Because you're scared" pointed out Izzy but before he could argue added "I think I would watch this. Clary?"

"After everyone else agrees then okay, why not?" said the other girl.

"Great! I'll download it!" said Izzy as she stormed out of the room.

Clary was left in the room only with the two boys and the dead silence was killing her.

"Um… should I go help Izzy?" she asked. She couldn't think about a single thing that would help Izzy download the movie but she felt too awkward.

"Forget it, I'll go." Said Alec as he went out of the room.

_Great_ – Clary thought. Staying alone with Jace didn't helped her either. They've kissed once but since Simon has interrupted them the situation between them could be explained only with the world 'cold'.

"So... um, I'm happy I'm not the only one without costume tonight" she spat, glaring at his usual for his 'job' black clothes. She realized this was probably the stupidest thing she could think of but hated how they were both just standing there, acting like they don't know each other.

Clary sometimes missed talking to Jace. She missed how he used to act like he doesn't care. So confident in everything he was doing. As he could never make a mistake. And Clary wanted to believe he is flawless. She wanted to believe she doesn't feel anything towards Jace Wayland thought there was something inside of her that was twisting every time he spoke.

"Clary, just don't" said Jace, taking her by surprise.

"What?" she asked him.

"Don't act like you really want to talk with me. That you're not doing it just because you want to kill the time while we wait."

"Jace…" she said, trying to reject the familiar twisting in her belly, making her feel guilty.

"What? You want to say that you actually want to talk with me?" he said and she could feel the bitterness her voice "Just think about what would your boyfriend say if he was here."

"Simon is not my boyfriend!" said Clary, suddenly feeling angry. How dared Jace mess somewhere, he didn't even had the right to.

"Movie is ready!" suddenly said Isabelle, walking in the room, holding a disk. Alec was walking just behind her.

She put the disk in the DVD player as she took the remote control and sat on the coach.

"And no more talking! The only sound allowed is screaming." Added the dark-haired girl.

Clary held the urge to roll her eyes, thought she felt kind of thankful towards the girl. She knew Izzy and Alec have probably heard her and Jace's argument and was glad they didn't comment it. However, she still felt like this wasn't her end of it. Sending a glare at Jace, who was still sitting on his place on the armchair, she could see that he also thought the same.

**A/N: I know, I know, they're a little OOC. Sorry about it. I tried to be as close to the real characters as I could but... **

**And don't worry, this won't be some kind of a dramatic, Mary-Sue-ish fic, this thing between Clary and Jace was just to show their "relationship" at this point but it would change later on. :D **

**P.S.: (Wow, my Author's notes are longer than the actual chapters :D) the other characters would come very soon. I would also like to include Tessa, Will, Jem...etc. but I don't know enough about them as I'm only at the beginning at TID and I'll screw everything. **

**P.P.S.: Just joking! :D This is the end :D **


	3. Scary movies and odd costumes

**A/N: Soo this is also very short but I decided to post more often but shorter chapters than once in a blue moon but well... not really much longer than this. **

**THANK YOU! THANK YOU! THANK YOU for the reviews and follows/favourites! :) **

**Important: Might contain spoilers for "Insidious :D " **

**Enjoy :D **

**Disclaimer: I do not own anything you already know.**

***Edit*: so I edited a little the end of this chapter as I had forgotten to add something I was planning to write. *facepalm* however, you might want to read the chapter since Simon and Maia's comming to the end so you won't be confused later. **

Most part of the movie was already over and Jace was losing patience now.

He didn't paid much atention to the screen as he had seen both parts of the movie already and it wasn't all that interesting or even scary. His mind was drifting somewhere between his last conversation with Clary and the urge to just leave the "party" and go sleep or find some demons to fight. He smiled at the thought. It would be much easier to just go out, go to fight, do the one thing he loved to do most and knew he is really good at. However, his smile faded as he cast a glance at the girl, sitting on the coach. Her hair was all around her face in a red mess and her green eyes showed fear and exitement in the same time as their gaze was fixed on the big screen right in front of her.

_How usual for the mundanes._ - Jace thought.

He always had thought they showed too much emotions, they were so easy to be read. But she wasn't a mundane, he remainded himself. He was the one to discover it, writing that healing rune on her althought there was a big chance of killing her if she was a normal human. But she wasn't. Clary was different from all the other people in so many ways, Jace thought. She wasn't extremely smart or outgoing or a great beauty but there was something in her. Something that was dragging the people towards her, he couldn't say for sure what it was but he knew he was also under the same spell. He didn't want to admit it but he felt so different in her company. Like he could tell her everything and that was so natural, the right thing to do.

As his eyes drifted to her face again he could say he was wrong. Not only she wasn't a mundane but even after looking at her for a hundred years, he still wouldn't be able to read her mind. Her eyes might showed emotion but her very thoughts were hidden deep inside.

"This movie is boring." He finally said, more because he wanted to distract his thoughts than to comment the horror movie he wasn't paying any atention to anyway.

"Hey, you agreed to watch it, now stay quiet" said Izzy, not removing her gaze from the TV as she reached her hand to grab some popcorns from the big bowl on the table.

"I just said that I don't mind it, not that I like it." Argued Jace.

"It's the same thing" said Alec.

"In the end boy returns to his body but the man stays trapped in the other world and a ghost of an old man that likes dressing like women comes on his place. There is a second part." muttered Jace, pissed off.

"Well thank you very much for spoiling the end!" Said Isabelle, sending him dead glares as she paused the movie.  
>"Hey, you all should be thankful I saved you from wasting your time on that thing" told her Jace.<p>

"Well, thanks to you, we probably won't need to. Hope you're not too surprised we're not very grateful." Said Alec.

"You are. You just don't know it yet" Jace said.

Before the argument could become even bigger, they were all interrupted by the bell of the Institute's door that rang loudly.

"It's probably Simon!" Said Clary.

Of course, thought Jace, who else could it be?

He wasn't sure why he didn't liked the boy so much but Simon also deffinitely didn't felt him as a best friend so the feelings were from both sides.

"I'll go open." Said Isabelle as she got up and exited the room.

After a few minutes, she returned with the said boy, and surprisingly, Maia next to him. However, Jace's gaze was fixed on Simon. He was wearing a really strange white costume with a white mask on his head. He was almost unrecognizable.

"Look who is here" said Izzy, smiling. "Simon also decided to bring someone." She didn't looked all that happy by the werewolf girl's appearance but she was hiding it good.

"Hey." Said Simon. "Oh, you're watching Insidious? In the end-"

"Yes, we know the end" Jace interrupted him.

"Jace was nice enough to spoil it for us" said Clary with sarcasm, glaring at the blonde boy. "Simon… what are you wearing!?" she then returned her gaze to her friend.

"I'm Boba Fett from Star Wars." Answered the boy, grinning. "Cool. Isn't it?"

Clary looked at him with a face, showing mixed feelings. "Um, actually there wasn't necessary to wear a costume Simon" she said.

"Oh, don't worry. I also wear one" said Izzy as she pointed at herself. "Probably it means that we're the only ones here that actually like this holiday." She added, glancing at the others. "

"Umm... hi, I guess" said Maia, obviously feeling uncomfortable from the whole situation but wanting to say something nonetheless.

"Hi, Maia" said Clary, turning to her.

_Always trying to be nice_ - Jace noted.

"So now as we are more, and I guess no one wants to watch the movie to it's end, what are we doing?" Asked Isabelle once again that night.

"Why not tell scary stories?" Asked Simon.

"Sure." Answered Izzy. "Who is 'for'?"

No one said or did anything so she tried again.

"Okaay, who is 'against'?" again no one did anything. "Well, I'm taking it as 'yes'. Everybody sit down"

As they all did as they were told, Simon with a little more efforts than the others as his costume obviously wasn't that comfortable, Isabelle went to turn off the lights, making the room look much more darker and mistic.

"Clary, you're first."


	4. Scary stories and ghosts

**A/N: ANDDD HAPPY HALLOWEEN EVERYONE! (at least in my country it's Halloween already). Hope y'all have amazing day and lots of fun^^**

**This chapter is bigger. Yayy :D Also, you might want to check the last one as I edited some parts. **

**Hope you enjoyy^^ **

**Question: How would you be dressed this Halloween? I'm going to be a Shadowhunter (didn't saw that coming, right? :D )**

**Disclaimer: I do not own any of the places or characters.**

"Me?" asked Clary surprised.

"Yeah, sure, go on." Said Izzy, smiling encouraging.

"But I don't know scary stories" answered the other girl. Of course she knew some of the old city myths and city legends that the adults used when trying to scare the children but Clary was afraid that those stories were too cliché and wouldn't impress the Shadowhunters and her other friends.

"Oh come on, Clary. Sure you know some good ones" said Simon, too.

"Yes, Clary, tell us a story" also added Jace.

"Okay… I guess I'll try tell one" said Clary finally as she tried to remember a good enough story that would at least save her from jokes. "It's it believed that this one really happened long ago. Once in a distant village lived a very old woman. Everyone thought her to be a witch but, however, there weren't any evidences for it. Suddenly, lots of people, mostly young, started disappearing from the village and their families were worried. There were often heard strange noises from the old lady's house at night, when most people were disappearing. Once one man decided to gather all the men in the village and as soon as the old woman was outside her house, they broke into it. As they researched the house, they found a locked door. As they broke it and went it, what they found was terrifying - there they found lots of bowls, cups and other dishes, full with red liquid, most likely blood. As soon as the woman returned, they all gathered around her and made her reveal what had she done. It soon came clear that she used to kill people and drink their blood in order to stay young. The woman was later burned, sentenced for witchcraft. The bodies of the dead were found buried in her garden and the families of the lost ones could finally have their peace. However, a lot of people say that they sometimes hear screams from the house, now abandoned, at night. The souls of the killed are looking for revenge." With this Clary finished her story.

"Now that was creepy." Said Simon, who, apparently haven't heard this one before.

"Is it really a true story?" asked Maia.

"I don't know" honestly answered Clary. She had heard stories like this from Madame Dorothea when she was a child but didn't believe at them much.

"Sounds like a story about one of those humans that try to mess into Downworlders' jobs." Commented Izzy.

"Well, it was a good story but hear this " said Jace as he started telling his story.

"Once on the place on this very Institute, there was an old cemetery."

"What? No, there wasn't!" Alec spoke for the first time as Simon and Maia arrived.

"How do you know that?" asked Jace, completely sure he was right.

"Because here was a church, everyone knows it" said Isabelle "Most of the Institutes are build on holy lands."

"Well, this one is not" argued Jace as he continued the story like he was never interrupted. "They say that it was a cemetery where only cursed people and criminals were buried."

"How can the Institute be build on a place on a place where they buried criminals before?!" asked Clary, too.

"Their souls couldn't find peace even when dead as they've never felt regret in their living lives." continued Jace, not noticing Clary's comment. "They wandered around the place day and night as shadows, neither living, nor completely dead. However, they couldn't leave the borders of the cemetery so all they could do is chase away everyone who neared. Soon people started seeking ways to drive the ghosts out so they would live in piece again. Somehow they even found a way to call a real warlock for help and he sent them away into Hell." With that Jace finished hid story and a dramatic pause was settled over the group.

"Is this all?" Finally asked Alec.

"He just sent them away. What's the scary in this?" asked Simon unbelievingly.

"Well, if you really want to know, there are rumors that the ghosts are appearing once in the year on Halloween night, exactly at midnight, when a portal is open between the worlds of the living and the dead." Answered them both Jace.

"Tonight?" asked Maia. "This sounds really…"

"Invented?" finished Clary for her.

"Well, believe it or not the ghosts love to come to the world and make jokes with people. They like throwing dishes, opening doors, or even ringing on the bells."

As he said this, there was a sudden ringing, coming from the door. Clary jumped a little, not really believing it's a ghost but getting startled anyways.

Jace grinned.

"Exactly midnight" he pointed out looking at the watch on the opposite wall.

"Oh, don't be silly!" said Isabelle as she got up and turned on the lights. "Those aren't ghosts. Just someone had decided to join us."

"Yes, you might be right about the second but how can you prove that exactly ghosts aren't the one that rang." Said Jace, still grinning.

"Isn't it believed that ghosts are immaterial? They can't really touch anything or harm anyone." Said Simon.

"And they can't came here because it's a holy land, which proves that your argument is invalid." Added Alec as there was another ringing.

"Someone better go open soon" said Maia.

"I'll go." said Clary, feeling the need to do something instead of just sitting there.

"I'll come with you. I'm the one that came up with the idea of the party and I need to handle with my guests" said Isabelle.

Clary smiled at her. She felt grateful that she wouldn't have to go all alone through the dark corridors of the Institute although she didn't believe in ghosts.

"Okay, the rest better stay here and decide what to do next. Oh, by the way, maybe we need some food, Jace, Alec, could you go to the kitchen and bring some?" said Izzy and before anyone could say anything more went out of the room, followed by Clary.

Outside the room was completely dark and the two girls had to pull out their witchlights in effort to see where they're going.

They went to take the elevator which disturbed the complete silence with it's noise as it came to stop in front of them. They weren't talking the way down, thought Clary wasn't finding the silence awkward at all. She felt strange to break the silence like that.

As the elevator stopped, the two girls went out of it and were greeted by a small, furry frame with two yellow eyes which were shining in the darkness.

"Hey, Church" said Izzy as she kneeled to pat the cat. Clary just smiled as the grey animal turned it's gaze to her. Then Church turned them his back as he went to the door. They followed him, as Clary stopped a few steps away from the door when Isabelle opened it.

On the doorway there were two boys and a girl. Clary has never seen them before but it looked as like Izzy knew them well.

The two boy were Shadowhunters, Clary could say, as their runes were showing on their uncovered hands. They were both tall, one of them with black hair, and as long as Clary could see, blue eyes. The other had grey hair, although they were both young. She could say that maybe he was half Chinese by his eyes, which were the same grey color as his hair. However, her gaze was on the girl. She looked to be on her age, thought unlike Clary she was very tall. She had brown hair and calm, grey eyes and looked beautiful. However, Clary couldn't understand what is she. A Downworlder?

"Hi!" said Izzy enthusiastic to the newcomers. "Glad you came tonight"

"Well, then you for the invitation" Said the boy with the grey hair.

"Very nice of you" added the girl smiling. The other boy just nodded.

"Well, what are we waiting for, come in." said Isabelle as she moved from the door and let the three inside.

Clary stayed there, feeling a little awkward and not knowing what to say.

"Oh, I almost forgot. This is Clary Fray" Said Isabelle pointing at Clary. "Clary, those are Tessa Gray, Will Herondale and Jem Carstairs. Will and Jem are Shadowhunters while Tessa is a Downworlder. However, they all live in the London Institute."

"Nice to meet you all" said Clary. She haven't heard of a Downworlder living in an Institute before but decided to keep it to herself. They looked as nice people.

"Nice to meet you, too." Said Tessa as the boys smiled. With a surprise Clary saw that Church, who usually wasn't very fond of people, was now stroking himself in Jem's feet.

"Now when you know each other, let's go to the others, shall we?" said Izzy as they all went to the elevator.

Once on the second floor, they quickly reached the room, the others were in. However, now only Simon and Maia were sitting on the couch.

After they met the newcomers and introduced each other, Isabelle asked.

"Where are Jace and Alec?"

"Oh, they went to make popcorns." Answered Simon.

"Oh, no. I know I can't cook well but I just hope they don't blow the whole kitchen." Said Isabelle, half-jokingly, half-serious.

"So did we interrupted something when we came?" asked Jem.

"Well, we were telling scary stories." Said Maia.

"Oh! Sounds great!" said Will then. "I know one really good…"

"Will," said Jem, warningly "Please just don't talk about Demon pox."

"Or ducks" added Tessa.

"What, how could you ever think about it?" he asked, acting as he was offended, thought no one bought it. "Okay then, but it's still a good story. Once near a park, there was a big lake with lots of ducks-" however, he never finished as there was a sudden explosion from downstairs and most particularly, from where the kitchen was.

"On, no!" said Isabelle as she hurried outside the room, followed by the others.

"Eh… you said something about kitchen and blowing earlier…" said Simon as they all hurried to the said place.

**A/N: Now Jace and Alec are dead :D **

**Tessa and the boys are here, yayy :D As it's an AU they aren't grand grand grand whatever parents to the characters and they aren't family realated but obviously know each other :D **

**About the stories: The story Clary tells is an old legend I've seen many times but a little changed and retelled. The story that Jace says, is obviously fully made up and a total lie and the story Will tries to tell, about the canibalistic ducks and the Demon pox, is obviously, completely true. **


End file.
